1. Field of the Invention
This invention relates generally to intraocular lenses and, more particularly, to intraocular lenses having fixation arms specially configured to facilitate proper implantation.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a normal human eye 10, which has a generally spherical body defined by an outer wall called the sclera 12 and a transparent bulbous front portion called the cornea 14. A lens 16 is located within the spherical body, behind the cornea, and an iris 18 is located between the lens and the cornea, dividing the eye into an anterior chamber 20 in front of the iris and a posterior chamber 22 in back of the iris. A central opening in the iris, called the pupil 24, controls the amount of light that reaches the lens. Light is focused by the cornea and the lens onto a retina 26 at the rear of the eye.
The lens 16 is a biconvex, highly transparent structure surrounded by Et thin lens capsule 28. The lens capsule is supported at its periphery by suspensory ligaments called zonules 30, which are continuous with the ciliary muscle 32. The ciliary muscle functions to change the focal length of the lens. Immediately in front of the zonules, between the ciliary muscle and iris 18, is a region referred to as the cilify sulcus 34.
A cataract condition results when the lens 16 becomes clouded, thereby obstructing the passage of light. To correct this condition, three alternative forms of surgery have been used, including intracapsular extraction, extracapsular extraction, and phacoemulsification. In intracapsular cataract extraction, the zonules 30 around the entire periphery of the lens capsule 28 are severed, and the entire lens structure, including the lens capsule, is then removed. In extracapsular cataract extraction and phacoemulsification, only the clouded material within the lens capsule is removed, while the transparent posterior lens capsule wall 28a with its peripheral portion, as well as the zonules, remain in the eye.
Intracapsular extraction, extracapsular extraction, and phacoemulsification all eliminate the light blockage caused by the cataract condition. However, light entering the eye thereafter is not properly focused onto the retina 26. A contact lens can be placed on the exterior surface of the eye, but this approach has the disadvantage that the patient has virtually no useful sight when the contact lens is removed. A preferred alternative is to implant an artificial lens, known as an intraocular lens (IOL), directly within the eye.
IOLs typically incorporate a disk-shaped, transparent lens optic and two smoothly curved attachment arms referred to as haptics. The lens optic typically is formed of a polymeric material such as polymethyl methacrylate, which is suitable for lathe turning or molding by injection, compression or cast molding techniques. The IOL is implanted through an incision made near the periphery of the cornea, which may be the same incision as is used to remove the cataract. The IOL is implanted either in the eye's anterior chamber, in front of the iris, or in the eye's posterior chamber, in back of the iris.
Anterior chamber IOLs can be supported by contact of the haptics with the iris 18 or with the anterior chamber angle, formed at the union of the iris and the cornea 14. Posterior chamber IOLs, on the other hand, can be supported by several alternative techniques. In one such technique, the IOL is placed in the sack-like structure formed by the intact posterior and peripheral walls of the lens capsule 28, where the haptics compress slightly against the periphery of the lens capsule, to hold the IOL in place. In another technique, the IOL is placed in front of and outside the lens capsule, where the haptics compress slightly against the ciliary sulcus 34. In yet another technique, the IOL is secured in place by suturing the haptics to the ciliary sulcus or to the iris.
The lens optics of many IOLs now are formed of materials that are sufficiently soft and flexible to allow the lens optic to be folded, for insertion into the eye through an incision of reduced size. After insertion, the lens is unfolded and positioned within the eye in a normal manner, e.g., with its haptics engaging the lens capsule, the ciliary sulcus, the iris, and/or the anterior chamber angle.
Many IOLs are asymmetric along their optical axes, perpendicular to the plane of the lens optic. For example, the haptics of posterior chamber IOLs frequently are angulated slightly in the anterior direction from the lens optic's plane. This angulation increases the spacing between the lens optic and the iris and thereby minimizes possible interference. In addition, the posterior and anterior surfaces of the IOLs typically are not identically shaped; each surface is uniquely shaped to achieve the desired optical correction.
Despite the asymmetry of many IOLs along their optical axes, it is sometimes difficult for the eye surgeon to visually differentiate the IOLs anterior and posterior sides. Consequently, the IOL occasionally can be improperly implanted in a reversed orientation within the eye, i.e., with its anterior side facing rearwardly and its posterior side facing forwardly. This reversal can adversely affect the optical correction provided by the IOL, and it can lead to problems associated with unwanted interaction with other eye tissue, including, for example, interference with the iris.
It should, therefore, be appreciated that there is a need for an IOL configured to minimize the possibility of improperly implanting it in a reverse orientation within the eye, with its anterior side facing rearwardly and its posterior side facing forwardly. The present invention fulfills this need.